Macrocyclic esters are obtained by the thermal depolymerization of the corresponding linear polyesters accompanied by ring closure. For such processes, the polyester is heated at an elevated temperature in the presence of a catalyst. The reaction is carried out under reduced pressure and the macrocyclic compound and other volatile products formed during the course of the depolymerization are removed from the reaction zone as they are formed.
Chlorides, nitrates, carbonates and oxides of magnesium, manganese, iron, cobalt and tin (all in the divalent state) are disclosed to be effective catalysts for the depolymerization of linear polyesters in U.S. Pat. No. 2,092,031. In the process of U.S. Pat. No. 4,165,321 Lewis metal salts such as the oxides hydroxides, halides, or carboxylates of Group IIIa, IVa, IVb, Va, VIIb and VIII metals are indictaed to be useful catalysts. Yasakawa et al. reported the use of lead catalysts (oxide, hydroxide, carbonate, nitrate, borate or organic acid salts) for the preparation of large ring lactones via thermal depolymerization in Chemical Abstracts, Vol. 78 (1973), 158966q and 158968s. Cyclic esters are also obtained via thermal degradation of polyesters using SnCl.sub.2. 2H.sub.2 O in Chemical Abstracts Vol. 86 (1977), 156163s. In U.S. Pat. Nos. 4,105,672, 4,136,098 and 4,157,330 tin carboxylates and organotin compounds are employed in conjunction with an 0,0-dialkyl-(3,5-di-t-butyl-4-hydroxybenzyl)phosphate to catalyze the reaction. In a somewhat related procedure, the formation of cyclic ester anhydrides of alpha-hydroxycarboxylic acids in vacuo by depolymerizing the corresponding linear polymer at 200.degree.-240.degree. C. in the presence of lead (II) stearate is described in British Patent No. 1,108,720.
In view of the problems associated with the use of heavy metal catalysts, aluminum oxide was suggested to catalyze depolymerizations carried out at atmospheric pressure using superheated steam in Czech Patent No. 108,762. The use of metallic aluminum was also reported in Japanese Patent No. 36-1375 (1961) for the thermal depolymerization of polyesters to form cyclic esters and lactones. Aluminum alcoholates are disclosed for the preparation of large-ring lactones in Japanese Patent Publication No. 72 25,071.
Mixed-metal catalysts having a carbonic acid radical and based on aluminum and sodium, wherein aluminum is the predominant metal, are disclosed for the depolymerization of polyesters to produce macrocyclic compounds in Japanese Patent Disclosure Nos. 1979-103,884 (appln. no. 1978-8,809); 1979-115,390 (appln. no. 1978-22,023); and 1980-120,581 (appln. no. 1979-26,741). The mixed-metal catalysts of these Japanese references all have aluminum:sodium weight ratios greater than about 3.5:1. The catalysts are typically prepared by treating an aqueous mixture of aluminum hydroxide and caustic soda with carbon dioxide.
It would be highly advantageous if other mixed-metal catalysts based on aluminum and sodium were available. It would be even more desirable if these new mixed-metal catalysts gave even faster reaction rates and if the catalysts were available from natural sources or could be synthetically produced.